1. Field of the Invention
The present invention relates to a zoom lens system for a micro-miniaturized digital camera which is used in a camera-equipped cellular phone, and in a PDA (Personal Digital Assistant), etc.
2. Description of the Prior Art
A digital camera which forms an image on an imaging device is not only used as a camera itself, but also is accommodated in a cellular phone and a PDA, etc. Accordingly, an attempt has been made to both miniaturize the zoom lens system and to increase the number of pixels (higher definition) at the same time.
If a zoom lens system for a conventional imaging device with a sparse pixel arrangement is used with an imaging device with a dense (higher definition) pixel arrangement, the required optical performance cannot be achieved even in the case where the size of an image formed by the conventional imaging device with a sparse pixel arrangement is the same as the size of an image formed by the imaging device with a dense pixels arrangement.
For example, in the prior art, an imaging device with a diagonal image height of 2.0 mm (y=2.0 mm), which has mainly been used in a cellular phone, has been able to be used with a zoom lens system having three lens elements. However, it has been understood that a higher-definition imaging device in recent years cannot achieve sufficient optical performance with the zoom lens system of the three lens elements. A sufficient optical performance can of course be achieved if the zoom lens system is constituted by a larger number of lens elements, e.g., seven lens elements. On the other hand, a zoom lens system with such a large number of lens elements inevitably makes the overall length thereof longer, and cannot be mounted in a small gadget like a cellular phone.